Semiconductor apparatus having nitride semiconductor buffer layer doped with at least one of Fe, Si, and C

ABSTRACT

A semiconductor apparatus includes a substrate; a buffer layer formed on the substrate; a first semiconductor layer formed on the buffer layer; and a second semiconductor layer formed on the first semiconductor layer. Further, the buffer layer is formed of AlGaN and doped with Fe, the buffer layer includes a plurality of layers having different Al component ratios from each other, and the Al component ratio of a first layer is greater than the Al component ratio of a second layer and a Fe concentration of the first layer is less than the Fe concentration of the second layer, the first and second layers being included in the plurality of layers, and the first layer being formed on a substrate side of the second layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority of theprior Japanese Patent Application No. 2012-218250, filed on Sep. 28,2012, the entire contents of which are incorporated herein by reference.

FIELD

The embodiment discussed herein is related to a semiconductor apparatus.

BACKGROUND

A nitride semiconductor such as GaN, AlN, InN and the like and amaterial of a mixed crystal thereof may have a wide band gap, so as tobe used as a high-power electronic device or a short-wavelengthlight-emitting device. Among these, research and development has beenconducted on the technologies of a Field-Effect Transistor (FET) andespecially a High Electron Mobility Transistor (HEMT) as a high-powerdevice (see, for example, Japanese Laid-open Patent Publication No.2002-359256).

The HEMT using such a nitride semiconductor is used in a high-power andhighly-efficient amplifier, a high-power switching device and the like.

For example, the band gap of gallium nitride (GaN), which is one of thenitride semiconductors, is 3.4 eV, which is greater than the band gap(1.1 eV) of Si and the band gap (1.4 eV) of GaAs, so that GaN has higherbreakdown field strength. As the HEMT using GaN, there is a HEMT inwhich the electron transit layer is formed of GaN, the electron supplylayer is formed of AlGaN, and an aluminum gallium nitride/galliumnitride (AlGaN/GaN) heterostructure is formed.

Due to the AlGaN/GaN heterostructure, a piezoelectric polarization maybe excited due to lattice distortion caused by a difference in latticeconstants between AlGaN and GaN. Due to the excited piezoelectricpolarization, in the GaN layer, highly-concentrated Two-DimensionalElectron Gas (2DEG) may be generated in a region near the interface(boundary surface) between the AlGaN layer and the GaN layer.

The HEMT using GaN is thought to be used especially in a high-efficiencyswitching device and a high withstand voltage power device to be usefulin an electric vehicle or the like. In such a high withstand voltagepower device, from a viewpoint of the circuit design, it is desired tosupport a normally-off operation.

SUMMARY

According to an aspect, a semiconductor apparatus includes a substrate;a buffer layer formed on the substrate; a first semiconductor layerformed on the buffer layer; and a second semiconductor layer formed onthe first semiconductor layer.

Further, the buffer layer is formed of AlGaN and doped with Fe.

Further the buffer layer includes a plurality of layers having differentAl component ratios from each other.

Further, the Al component ratio of a first layer is greater than the Alcomponent ratio of a second layer and a Fe concentration of the firstlayer is less than the Fe concentration of the second layer, the firstand second layers being included in the plurality of layers, and thefirst layer being formed on a substrate side of the second layer.

The objects and advantages of the embodiments disclosed herein will berealized and attained by means of the elements and combinationsparticularly pointed out in the claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention as claimed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a HEMT using GaN in related art;

FIG. 2 illustrates a relationship between a concentration distributionof Fe and characteristic items;

FIG. 3 illustrates an example configuration of a semiconductor apparatusaccording to a first embodiment;

FIG. 4 is a correlation diagram between a V/III ratio and Twist of a GaNlayer when an AlN layer is formed;

FIGS. 5A and 5B illustrate a leakage current of the semiconductorapparatus according to the first embodiment;

FIGS. 6A and 6B illustrate steps in a method of manufacturing thesemiconductor apparatus according to the first embodiment;

FIGS. 7A and 7B illustrate other steps in the method of manufacturingthe semiconductor apparatus according to the first embodiment;

FIG. 8 illustrates a modified example configuration of a semiconductorapparatus according to the first embodiment;

FIG. 9 illustrates another modified example configuration of asemiconductor apparatus according to the first embodiment;

FIG. 10 illustrates still another modified example configuration of asemiconductor apparatus according to the first embodiment;

FIGS. 11A and 11B illustrate example configurations of othersemiconductor apparatuses according to the first embodiment;

FIG. 12 illustrates a semiconductor apparatus according to a secondembodiment;

FIG. 13 illustrates a semiconductor apparatus according to a thirdembodiment;

FIG. 14 is an example energy band diagram of a semiconductor apparatusaccording to the third embodiment;

FIG. 15 illustrates a semiconductor apparatus according to a fourthembodiment;

FIG. 16 illustrates a semiconductor apparatus according to a fifthembodiment;

FIG. 17 illustrates a semiconductor apparatus according to a sixthembodiment;

FIG. 18 illustrates a semiconductor apparatus according to a seventhembodiment;

FIG. 19 illustrates a discretely-packaged semiconductor device accordingto an eighth embodiment;

FIG. 20 illustrates an example circuit diagram of a power-supplyapparatus according to the eight embodiment; and

FIG. 21 illustrates an example configuration a high-power amplifieraccording to the eighth embodiment.

DESCRIPTION OF EMBODIMENT

In manufacturing HEMTs using a nitride semiconductor, a siliconsubstrate which is an inexpensive and large-size substrate is typicallyused due to the requirement of lower cost. However, there are greatdifferences in the lattice constant and the heat expansion coefficientbetween silicon and a nitride semiconductor material.

Therefore, generally, a buffer layer is formed on the silicon substrateand the nitride semiconductor such as GaN to form a HEMT is formed onthe buffer layer.

Specifically, as illustrated in FIG. 1, in the above described HEMT, abuffer layer 920 formed of AlGaN is formed on a silicon substrate 910,and an electron transit layer 931 formed of GaN and an electron supplylayer 932 are sequentially formed (laminated) on the buffer layer 920.Further, a gate electrode 941, a source electrode 942, and a drainelectrode 943 are formed on the electron supply layer 932

In this case, generally, the buffer layer 920 is formed on the siliconsubstrate 910 so that the buffer layer 920 has a greater thickness. Whena transistor is formed on the buffer layer 920, it may be difficult tocontrol an operation between the source and the drain based on voltagecontrol at the gate electrode 941. Further, a leakage current passingthrough the buffer layer 920 may occur as illustrated in the arrow 950b.

Further, to drive the transistor, a positive bias voltage is applied tothe drain electrode 942. However, in this case, as illustrated in thearrow 950 b, a leakage current occurs that flows to the rear side of thesilicon substrate 910 via the electron supply layer 932, the electrontransit layer 931, and the buffer layer 920.

The leakage current may depend on remaining carriers in the electronsupply layer 932, the electron transit layer 931, and the buffer layer920. Namely, the leakage current amount varies depending on the amountof such remaining carriers.

When the leakage current is large, the power loss is increased andenergy consumption is accordingly increased. To resolve the problem, itis desired to provide a semiconductor apparatus using a nitridesemiconductor such as GaN as a semiconductor material having lowerleakage current.

In the following, embodiments to carry out the invention are described.Here, the same reference numerals are used to describe the sameelements, and the repeated descriptions thereof may be omitted.

First Embodiment

To reduce the above-described leakage current, there is a method toincrease the resistance of the nitride semiconductor layer such as thebuffer layer 920 by doping, with impurity element such as Fe, thenitride semiconductor layer. However, when the impurity element such asFe as a dopant is injected (doped) into the nitride semiconductor layersuch as the buffer layer 920, the crystal property may be degraded andthe on-resistance value may be increased.

FIG. 2 illustrates relationships between Fe concentration distributionwhen Fe is injected into the buffer layer and the electron transit layerand evaluation items which are withstand, crystal property and Collapse.The term “crystal property” herein refers to the crystal property in thenitride semiconductor including the electron transit layer and the like.The term “Collapse” herein refers to a current collapse phenomenon wherethe on-resistance value in a higher voltage operation becomes greaterthan that in a lower voltage operation.

In FIG. 2, as illustrated in a case “2A” where Fe as a dopant isuniformly injected in higher concentration into the buffer layer and theelectron transit layer, the withstand voltage is high, but the crystalproperty is not good, so that the on-resistance becomes high, and thecollapse may occur. Further, in a case “2B” where Fe is substantiallyuniformly injected in higher concentration into the buffer layer and Fedope amount is gradually decreased in the electron transit layer, thewithstand voltage is relatively high, but the crystal property is notgood so that the on-resistance is high and the control of the collapseis not sufficient.

Further, in a case “2C” where Fe dope amount is gradually increased inthe buffer layer and is gradually decreased in the electron transitlayer, the withstand voltage is relatively high and the crystal propertyis good so that the on-resistance is low and the collapse issufficiently controlled.

The crystal property in the electron transit layer and the like mayheavily depend on the crystal property of the buffer layer formed on thesubstrate. Therefore, the crystal property of the electron transit layerwhere the Fe dope amount is gradually increased is better than thecrystal property of the electron transit layer where the Fe is uniformlyinjected in high concentration.

Further, if Fe is included in a region where electrons transit in theelectron transit layer, the on-resistance value is accordingly increasedand the collapse becomes remarkable. Therefore, it is preferable that Feas a dopant is not injected into the region where electrons transit inthe electron transit layer. This embodiment is achieved based on thisconsideration.

Semiconductor Apparatus

A semiconductor apparatus according to the first embodiment is describedwith reference to FIG. 3. As illustrated in a part (a) of FIG. 3, thesemiconductor apparatus in this embodiment includes nitridesemiconductor layers, which are a nucleation layer 11, a buffer layer20, an electron transit layer (“a first semiconductor layer”), anelectron supply layer 32 (“a second semiconductor layer”) and the like,which are sequentially formed (laminated) on a substrate 10. Further, agate electrode 41, a source electrode 42, and a drain electrode 43 areformed on the electron supply layer 32.

As the substrate 10, a substrate formed of Si, SiC, sapphire, GaN or thelike may be used. The nitride semiconductor layers such as thenucleation layer 11, the buffer layer 20, the electron transit layer 31,the electron supply layer 32 and the like may be formed by Metal OrganicVapor Phase Epitaxy (MOVPE) or Molecular Beam Epitaxy (MBE). In thisembodiment, those nitride semiconductor layers are formed by MOVPE.

The nucleation layer 11 is formed of an AlN layer having a thickness ofapproximately 200 nm. In this embodiment, Fe as a dopant is injectedinto the nucleation layer 11 with a concentration of approximately5×10¹⁷ cm⁻³. However, Fe may not be injected into the nucleation layer11. Further, in this embodiment, the nucleation layer 11 may bedescribed as a part of the buffer layer 20.

The buffer layer 20 is formed of a first buffer layer 21, a secondbuffer layer 22, and a third buffer layer 23. In this embodiment, thecomposition of the first buffer layer 21 is Al_(X)Ga_(1-x)N, thecomposition of the second buffer layer 22 is Al_(Y)Ga_(1-Y)N, and thecomposition of the third buffer layer 23 is Al_(Z)Ga_(1-Z)N, so that therelationship 1>X>Y>Z>0 is satisfied. Specifically, the first bufferlayer 21 is formed of Al_(0.8)Ga_(0.2)N, the second buffer layer 22 isformed of Al_(0.5)Ga_(0.5)N, and the third buffer layer 23 is formed ofAl_(0.2)Ga_(0.8)N.

As described, the first buffer layer 21, the second buffer layer 22, andthe third buffer layer 23 are formed in a manner such that thecomposition ratio of Al is sequentially decreased as the distance to thesubstrate 10 is increased. In this embodiment, the term “compositionratio of Al” refers to a ratio of (the number of) Al atoms to the sum of(the numbers of) Al atoms and Ga atoms. Also, the values X, Y, and Zrefers to the corresponding “composition rates of Al”.

Further, as illustrated in a part (b) of FIG. 3, the buffer layer 20 isformed in a manner that a formula (Fe concentration in first bufferlayer 21)<(Fe concentration in second buffer layer 22)<(Fe concentrationin third buffer layer 23) is satisfied. Specifically, Fe is injectedinto the first buffer layer 21 with a concentration of approximately1×10¹⁸ cm⁻³, Fe is injected into the second buffer layer 22 with aconcentration of approximately 5×10¹⁸ cm⁻³, and Fe is injected into thethird buffer layer 23 with a concentration of approximately 1×10¹⁹ cm⁻³.

By doing this, Fe concentration is increased as the distance between thelayer and the substrate is increased. Here, preferably, the Feconcentration is greater than or equal to 5×10¹⁶ cm⁻³ from a viewpointof preventing the generation of carriers and less than or equal to2×10²⁰ cm⁻³ from a viewpoint of preventing the degradation of thecrystal property.

Further, the electron transit layer 31 is formed of GaN, and theelectron supply layer 32 is formed of AlGaN. By doing this, in theelectron transit layer 31, a 2DEG 31 a is formed in a region near theinterface between the electron transit layer 31 and the electron supplylayer 32.

Further, in this embodiment, Fe is injected into the region in theelectron transit layer 31 near the interface between the electrontransit layer 31 and the buffer layer 20 with a concentration ofapproximately 2×10¹⁹ cm⁻³. Specifically, in the electron transit layer31, Fe is injected into a region, which includes the interface betweenthe electron transit layer 31 and the buffer layer 20 and has athickness of approximately 200 nm, with a concentration greater than theFe concentration in the third buffer layer 23.

Fe is not injected into the entity of the electron transit layer 31.Namely, as described above, it is preferable that the Fe concentrationin the region near the interface between the electron transit layer 31and the electron supply layer 32 is substantially zero (none). Further,as described above, the gate electrode 41, the source electrode 42, andthe drain electrode 43 are formed on the electron supply layer 32.

In this embodiment, the buffer layer 20 is formed in a manner that thecomposition ratio of Al is decreased in the layer arranging order of thefirst buffer layer 21, the second buffer layer 22, and the third bufferlayer 23 as the distance to the substrate 10 is increased.

Further, the buffer layer 20 is formed in a manner that Fe concentrationis increased in the layer arranging order of the first buffer layer 21,the second buffer layer 22, and the third buffer layer 23 as thedistance to the substrate 10 is increased. Namely, the buffer layer 20is formed in a manner that the lower the composition ratio of Albecomes, the greater the Fe concentration becomes.

Here, with reference to FIG. 4, a relationship between the impurityconcentration and the crystal property in the nitride semiconductor isdescribed. FIG. 4 illustrates a relationship between a V/III ratio whenthe AlN layer is formed and Twist in the GaN layer formed on the AlNlayer. Here, the term “V/III ratio” refers to a ratio of a group-Vsource gas to a group-III source gas supplied when the AlN layer isformed by MOVPE.

By changing the V/III ratio upon forming the AlN layer, theconcentration of C (“C concentration”) included in the AlN layer may bechanged. Therefore, based on the relationship of FIG. 4, it may becomepossible to acquire the C concentration in the AlN layer and the Twistin the GaN layer.

Generally, the lower the V/III ratio is, the higher the C concentrationbecomes in the formed AlN layer. Further, the higher the V/III ratio is,the lower the C concentration becomes in the formed AlN layer. The term“Twist” in the GaN layer refers to an index indicating a crystaldistortion (twist) in the GaN layer and corresponds to a half-band widthby the X-ray diffraction.

As the Twist value of the GaN layer is increased, the crystal propertyis degraded, which may cause a defect level and electron scattering, sothat the collapse may become remarkable. Therefore, it is desired thatthe Twist value of the GaN layer is as low as possible.

When the C concentration in the AlN layer is high, dislocation or thelike may frequently occur. As a result, in epitaxial growth, the growthis performed while the dislocations remain. Therefore, as illustrated inFIG. 4, it is supposed that the crystal property may be degraded.

From the viewpoint of the impurity element, C may be thought to besimilar to Fe. Therefore, if the concentration of the impurity elementin the AlN layer is high, it is thought that the crystal property of theGaN layer is degraded. Therefore, it is desired that the concentrationof the impurity element in the nucleation layer 11 and the buffer layer20 be as low as possible.

Specifically, among the cases illustrated in FIG. 2, it may becomepossible to form the GaN layer having better crystal property in thecase 2C rather than in the cases 2A and 2B. Further, the crystalproperty may be remarkably degraded when an impurity element is injectedinto a layer having a higher composition ratio of Al.

Next, with reference to FIGS. 5A and 5B, the leakage current to the rearsurface of the substrate 10 is described. FIG. 5A illustrates a casewhere a voltage is applied to the layers laminated to each otherincluding an AlN layer 911, a first AlGaN layer 921, a second AlGaNlayer 922, a third AlGaN layer 923, and an electron transit layer 931where Fe as a dopant is not injected into any of the layers. When Fe isnot injected, the band gap of the nitride semiconductor having a lowercomposition ratio of Al is narrow.

Therefore, remaining carriers due to impurity element or defect are morelikely to be generated to become n-type. Accordingly, as illustrated inFIG. 5A, an electric field is concentrated on the layers from the AlNlayer 911, to the second AlGaN layer 922 or the like, and the bending ofthe band gets steep. As a result, a tunnel current becomes more likelyto flow and the leakage current becomes more likely to flow as well.

On the other hand, in this embodiment, as illustrated in FIG. 5B, theelectric field is uniformly formed. Therefore, there is no part wherethe bending of the band gets steep. Therefore, the tunnel current isunlikely to flow, so that the leakage current may be reduced. Here, itshould be noted that the electric field distributions illustrated inFIGS. 5A and 5B where the layers have the same band gap are forexplanatory purposes only.

As described above, in the semiconductor apparatus according to thisembodiment, it may become possible to reduce (prevent) the leakagecurrent that flows between the source and drain and to the rear surfaceof the substrate while preventing the degradation of the crystalproperty. Further, as will be described below, the semiconductorapparatus in this embodiment may have a structure where a recess isformed just under the gate electrode 41 or a p-type GaN layer and thelike are included, so as to support the normally-off operation.

Method of Manufacturing Semiconductor Apparatus

Next, a method of manufacturing the semiconductor apparatus according tothe first embodiment is described with reference to FIGS. 6A and 6B.

First, as illustrated in FIG. 6A, the nucleation layer 11 is formed onthe substrate 10 by epitaxial growth using MOVPE. In this embodiment,the nucleation layer 11 is formed of an AlN layer having a thickness ofapproximately 200 nm. Further, Fe as the impurity element is injectedinto the nucleation layer 11 with a concentration of approximately5×10¹⁷ cm⁻³.

Next, as illustrated in FIG. 6B, the buffer layer 20 is formed on thenucleation layer 11 by epitaxial growth using MOVPE. The buffer layer 20is formed of the first buffer layer 21, the second buffer layer 22, andthe third buffer layer 23, so that the entire thickness is approximately500 nm.

In this embodiment, the first buffer layer 21 is formed ofAl_(0.8)Ga_(0.2)N, and Fe as the impurity element is injected into thefirst buffer layer 21 with a concentration of approximately 1×10¹⁸ cm⁻³.

The second buffer layer 22 is formed of Al_(0.5)Ga_(0.5)N, and Fe as theimpurity element is injected into the second buffer layer 22 with aconcentration of approximately 5×10¹⁸ cm⁻³. The third buffer layer 23 isformed of Al_(0.2)Ga_(0.8)N, and Fe as the impurity element is injectedinto the third buffer layer 23 with a concentration of approximately1×10¹⁹ cm⁻³.

Next, as illustrated in FIG. 7A, the electron transit layer 31 and theelectron supply layer 32 are formed on the buffer layer 20 by epitaxialgrowth using MOVPE. The electron transit layer 31 (“first semiconductorlayer”) is formed of GaN layer having a thickness of approximately 1 μm.The electron supply layer 32 (“second semiconductor layer”) is formed ofAlGaN layer having a thickness of approximately 20 nm.

Further, Fe as the impurity element is injected into a region in theelectron transit layer 31 with a concentration of approximately 2×10¹⁹cm⁻³, the region being near (in contact with) the interface, which isbetween the electron transit layer 31 and the buffer layer 20, andhaving a thickness of approximately 200 nm. On the other hand, the Feconcentration in a region near the interface between the electrontransit layer 31 and the electron supply layer 32 is less than or equalto 1×10¹⁷ cm⁻³.

In the epitaxial growth by MOVPE, as the source gas of Al, trimethylaluminum (TMA) is used. As the source gas of Ga, trimethyl gallium (TMG)is used. As the source gas of N, ammonia (NH₃) is used. Further, as thesource gas of Fe, biscyclopentadienyl iron (Cp2Fe) is used. Further, thesource gas is supplied to a chamber of the MOVPE apparatus along withhydrogen (H₂) serving as carrier gas.

When the nucleation layer 11 is formed, the pressure in the chamber isset to 5 kPa, the growth temperature is set to 1,000° (degrees Celsius),and the group-III source gas and the group-V source gas are suppliedwith the V/III ratio (the ratio of a group-V source gas to a group-IIIsource gas) at a range from 1,000 to 2,000.

When the buffer layer 20 is formed, the pressure in the chamber is setto 5 kPs, the growth temperature is set to 1,000° (degrees Celsius), andthe group-III source gas and the group-V source gas are supplied withthe V/III ratio at a range from 100 to 600. In this case, apredetermined amount of Cp2Fe, which serves as the source gas to dopewith Fe, is supplied.

When the electron transit layer 31 is formed, before the Fe is injectedinto the first 200 nm, the pressure in the chamber is set to 10 kPa, thegrowth temperature is set to 1,000° (degrees Celsius), and the group-IIIsource gas and the group-V source gas are supplied with the V/III ratioless than or equal to 100.

In this case, a predetermined amount of Cp2Fe, which serves as thesource gas to dope with Fe, is supplied at the same time. After that,the supply of Cp2Fe is stopped. Next, when the electron transit layer 31is further formed, the pressure in the chamber is set to 20 kPa, thegrowth temperature is set to 1,000° (degrees Celsius), and the group-IIIsource gas and the group-V source gas are supplied with the V/III ratiogreater than or equal to 3,000.

When the electron supply layer 32 is formed, the pressure in the chamberis set to 20 kPa, the growth temperature is set to 1,000° (degreesCelsius), and the group-III source gas and the group-V source gas aresupplied with the V/III ratio greater than or equal to 3,000.

Next, as illustrated in FIG. 7B, the gate electrode 41, the sourceelectrode 42, and the drain electrode 43 are formed on the electronsupply layer 32.

Specifically, first, a photoresist is applied on the electron supplylayer 32. Then, by performing an exposition and development processusing an exposure device, the resist pattern is formed having openingscorresponding to the regions where the source electrode 42 and the drainelectrode 43 are to be formed. After that, a metal laminate film formedof Ti/Al (film thickness: Ti: 100 nm and Al: 300 nm) is formed by vacuumevaporation.

Then, the resist pattern and the metal laminate film formed on theresist pattern are dipped in organic solvent so as to be removed (liftedoff). By doing this, the source electrode 42 and the drain electrode 43are formed using the remaining metal laminate film. Then, rapid thermalannealing (RTA) is performed at a temperature of approximately 600°(degrees Celsius) to form ohmic contact.

Further, after that, another photoresist is applied on the electronsupply layer 32. Then, by performing the exposition and developmentprocess using the exposure device, the resist pattern is formed havingan opening corresponding to a region where the gate electrode 41 is tobe formed.

After that, a metal laminate film formed of Ni/Au (film thickness: Ni:50nm and Au:300 nm) is formed by vacuum evaporation. Then, the resistpattern and the metal laminate film formed on the resist pattern aredipped in organic solvent so as to be removed (lifted off). By doingthis, the gate electrode 41 is formed using the remaining metal laminatefilm.

By doing this, the semiconductor apparatus according to this embodimentmay be manufactured.

Modified Example

Next, modified examples in this embodiment are described. First, asillustrated in FIG. 8, in the semiconductor apparatus in thisembodiment, the buffer layer 20 may include two layers (i.e., a firstbuffer layer 21 and a second buffer layer 22) having different componentratios of Al (“Al component ratios”) and Fe concentration ratios fromeach other. As described above, the buffer layer 20 is formed in amanner that the first buffer layer 21 has higher component ratio of Al(“Al component ratio”) and lower Fe concentration than those of thesecond buffer layer 22.

Even in this configuration, it may become possible to lower the leakagecurrent while preventing the degradation of the crystal property.Further, in the semiconductor apparatus according to this embodiment,the buffer layer 20 may be formed of a plurality of layers having thesame component ratio of Al with each other and different Feconcentration ratios from each other.

Next, as illustrated in FIG. 9, in the semiconductor apparatus accordingto this embodiment, Fe as a dopant may be injected into the buffer layer20 but practically Fe may not be injected into the electron transitlayer 31. Even in this configuration, it may become possible to lowerthe leakage current while preventing the degradation of the crystalproperty.

Further, in the semiconductor apparatus according to this embodiment,the buffer layer 20 may be formed of a plurality of layers havingdifferent Fe concentration ratios from each other. Further, due todiffusion of Fe from the buffer layer 20 to the electron transit layer31, there may exist diffused Fe in a region of the electron transitlayer 31 near the interface between the electron transit layer 31 andthe buffer layer 20.

Further, as illustrated in FIG. 10, in the semiconductor apparatus inthis embodiment, when the Fe is injected into a part of the electrontransit layer 31, the buffer layer 20 may be formed in a manner that theFe concentration in the buffer layer 20 may be constant.

In this case, the buffer layer 20 is formed in a manner that theconstant Fe concentration in the buffer layer 30 is less than the Feconcentration in a region of the electron transit layer 31 near theinterface between the buffer layer 30 and the electron transit layer 31.Even in this configuration, there exist a plurality of layers havingdifferent Fe concentration ratios from each other. Therefore, the effectaccording to this embodiment may also be acquired.

Further, as illustrated in FIG. 11A, a recess 51 may be formed justunder the gate electrode 41 by removing a part of the electron supplylayer 32, so that the 2DEG 31 a is eliminated just under the gateelectrode 41 to achieve a normally-off operation.

Further, as illustrated in FIG. 11B, a p-GaN layer 52 may be formedbetween the electron supply layer 32 and the gate electrode 41. By doingthis, similarly, it may become possible to eliminate the 2DEG 31 a justunder the gate electrode 41 and achieve a normally-off operation.

Second Embodiment

Next, a second embodiment is described. As illustrated in FIG. 12, in asemiconductor apparatus according to the second embodiment, thenucleation layer 11, the first buffer layer 21, the second buffer layer22, the third buffer layer 23, the electron transit layer 31 and thelike are formed in a manner that the Fe concentration is higher in theinterfaces between the nucleation layer 11 and the first buffer layer21, between the first buffer layer 21 and the second buffer layer 22,between the second buffer layer 22 and the third buffer layer 23, andbetween the third buffer layer 23 and the electron transit layer 31.

In the interface between two layers having different component ratios ofAl from each other, due to difference in lattice constants, carriers arehighly likely to be generated. Therefore, by increasing the Feconcentration in such regions where carriers are highly likely to begenerated as described above, the carrier generation may be prevented.This is because, in this embodiment, the regions including the interfacebetween two layers having different component ratios of Al and where Feis doped with higher concentration are formed.

In this embodiment, the nucleation layer 11 is formed of AlN which isdoped with Fe at a concentration of approximately 5×10¹⁷ cm⁻³. The firstbuffer layer 21 is formed of Al_(0.8)Ga_(0.2)N with Fe concentration ofapproximately 1×10¹⁸ cm⁻³. There is formed a region 21 a including theinterface between the nucleation layer 11 and the first buffer layer 21.Fe is injected into the region 21 a with a concentration ofapproximately 1×10¹⁹ cm⁻³. The thickness of the region 21 a doped withhigher concentration of Fe is in a range from 30 nm to 50 nm.

The second buffer layer 22 is formed of Al_(0.5)Ga_(0.5)N with Fe dopantconcentration of approximately 5×10¹⁸ cm⁻³. There is formed a region 22a including the interface between the first buffer layer 21 and thesecond buffer layer 22. Fe is injected into the region 22 a with aconcentration of approximately 5×10¹⁹ cm⁻³. The thickness of the region22 a doped with a higher concentration of Fe is in a range from 30 nm to50 nm.

The third buffer layer 23 is formed of Al_(0.2)Ga_(0.8)N with Fe dopantconcentration of approximately 1×10¹⁹ cm⁻³. There is formed a region 23a including the interface between the second buffer layer 22 and thethird buffer layer 23. Fe is injected into the region 23 a with aconcentration of approximately 1×10²⁰ cm⁻³. The thickness of the region23 a doped with a higher concentration of Fe is in a range from 30 nm to50 nm.

Further, there is formed a region in the electron transit layer 31. Thethickness of the region is approximately 200 nm. Further, the regionincludes (started from) the interface between the electron transit layer31 and the buffer layer 20. Fe is injected into the region with aconcentration of approximately 2×10¹⁹ cm⁻³.

Further, there is formed a region 31 a including the interface betweenthe electron transit layer 31 and the buffer layer 20. Fe is injectedinto the region 31 a with a concentration of approximately 2×10²⁰ cm⁻³.The thickness of the region 23 a doped with a higher concentration of Feis in a range from 30 nm to 50 nm.

As a method of increasing the Fe concentration in the interfaces betweenlayers, a supply amount of the source gas for Fe may be increased at thetimings when the corresponding interfaces are formed. By doing this, thesemiconductor apparatus in this embodiment may be manufactured. All thesteps in this embodiment other than the above are the same as those inthe first embodiment.

Third Embodiment

Next, a third embodiment is described. In the semiconductor apparatus inthis embodiment, as illustrated in FIGS. 13 and 14, Fe concentration isset higher in a region which starts from the interface between twolayers and extends only in the layer having a narrower band gap. Thereare regions 21 b, 22 b, 23 b, and 31 b which are formed from theinterfaces formed between adjacent two layers including the nucleationlayer 11, the first buffer layer 21, the second buffer layer 22, thethird buffer layer 23, the electron transit layer 31 and the like.

Specifically, those layers are formed in a manner that Fe concentrationis higher in the first buffer layer 21 on the nucleation layer 11 side,in the second buffer layer 22 on the first buffer layer 21 side, in thethird buffer layer 23 on the second buffer layer 22 side, and in theelectron transit layer 31 on the third buffer layer 23 side.

As described above, at the interface between two layers having differentcomponent ratios of Al, due to the difference in lattice constantstherebetween, carriers are likely to be generated. Especially, carriersare more likely to be generated on the side of the layer whose band gapis narrower.

Therefore, by increasing the Fe concentration in the region where thecarriers are more likely to be generated, the carrier generation may bemore effectively prevented. To that end, in this embodiment, the regionshaving higher Fe dopant concentration are formed near the interface inthe layer having narrower band gap.

In this embodiment, the nucleation layer 11 is formed of AlN which isdoped with Fe at a concentration of approximately 5×10¹⁷ cm⁻³. The firstbuffer layer 21 is formed of Al_(0.8)Ga_(0.2)N with Fe dopedconcentration of approximately 1×10¹⁸ cm⁻³. In the first buffer layer21, in the vicinity of the interface between the nucleation layer 11 andthe first buffer layer 21, there is formed a region 21 b where Fe ishighly doped with a concentration of 1×10¹⁹ cm⁻³. The thickness of theregion 21 b is in a range from 30 nm to 50 nm.

Further, the second buffer layer 22 is formed of Al_(0.5)Ga_(0.5)N withFe dopant concentration of approximately 5×10¹⁸ cm⁻³. In the secondbuffer layer 22, in the vicinity of the interface between the firstbuffer layer 21 and the second buffer layer 22, there is formed a region22 b that is highly doped with Fe at a concentration of 5×10¹⁹ cm⁻³. Thethickness of the region 22 b is in a range from 30 nm to 50 nm.

Further, the third buffer layer 23 is formed of Al_(0.2)Ga_(0.8)N withFe dopant concentration of approximately 1×10¹⁹ cm⁻³. In the thirdbuffer layer 23, in the vicinity of the interface between the secondbuffer layer 22 and the third buffer layer 23, there is formed a region23 b that is highly doped with Fe at a concentration of 1×10²⁰ cm⁻³. Thethickness of the region 23 b is in a range from 30 nm to 50 nm.

Further, in the electron transit layer 31, there is a region whichstarts from the interface between the buffer layer 20 and the electrontransit layer 31, and extends in the electron transit layer 31. Thethickness of this region is approximately 200 nm. This region is dopedwith a Fe concentration of approximately 2×10¹⁹ cm⁻³. Further, this isanother region 31 b in the electron transit layer 31.

The region 31 b also starts from the interface between the buffer layer20 and the electron transit layer 31, and extends in the electrontransit layer 31. The region 31 b is highly doped with a Feconcentration of 2×10²⁰ cm⁻³. The thickness of the region 31 b is in arange from 30 nm to 50 nm.

As a method of forming the regions highly doped with Fe, for example,there is a method in which the supply amount of the source gas for Fe isincreased at each of the timings when new layers are formed havingdifferent composition ratios from those before. By doing this, thesemiconductor apparatus in this embodiment may be manufactured. All thesteps in this embodiment other than the above are the same as those inthe first embodiment.

Fourth Embodiment

Next, a fourth embodiment is described. In the semiconductor apparatusaccording to this embodiment, as illustrated in FIG. 15, not only Fe butalso Si are injected as dopants in the fourth embodiment.

As described above, at the interface between two layers having differentcomponent ratios of Al, due to the difference in lattice constantstherebetween, carriers are likely to be generated. Generally, thecarriers that are generated in this case are electrons.

However, holes may be generated. Silicon (Si) is an n-type impurityelement. Therefore, in this embodiment, by doping with Si, thegeneration of holes may be prevented. However, if Si doping is too much,the excess holes may become sources to supply electrons. Therefore,preferably, Si dopant concentration be less than or equal to 2×10¹⁷cm⁻³.

In this embodiment, the nucleation layer 11 is formed of AlN with Fedopant concentration of approximately 5×10¹⁷ cm⁻³. The first bufferlayer 21 is formed of Al_(0.8)Ga_(0.2)N with Fe doped concentration ofapproximately 1×10¹⁸ cm⁻³.

In the first buffer layer 21, in the vicinity of the interface betweenthe nucleation layer 11 and the first buffer layer 21, there is formed aregion 21 c that is doped with a Fe concentration of 1×10¹⁹ cm⁻³ and isdoped with a Si concentration of 2×10¹⁷ cm⁻³ or less. The thickness ofthe region 21 c is in a range from 30 nm to 50 nm.

Further, the second buffer layer 22 is formed of Al_(0.5)Ga_(0.5)N withFe dopant concentration of approximately 5×10¹⁸ cm⁻³. In the secondbuffer layer 22, in the vicinity of the interface between the firstbuffer layer 21 and the second buffer layer 22, there is formed a region22 c that is doped with a Fe concentration of approximately 1×10¹⁹ cm⁻³and is doped with a Si concentration of 2×10¹⁷ cm⁻³ or less. Thethickness of the region 22 c is in a range from 30 nm to 50 nm.

Further, the third buffer layer 23 is formed of Al_(0.2)Ga_(0.8)N withFe dopant concentration of approximately 1×10¹⁹ cm⁻³. In the thirdbuffer layer 23, in the vicinity of the interface between the secondbuffer layer 22 and the third buffer layer 23, there is formed a region23 c that is doped with a Fe concentration of 1×10¹⁹ cm⁻³ and is dopedwith a Si concentration of 2×10¹⁷ cm⁻³ or less. The thickness of theregion 23 c is in a range from 30 nm to 50 nm.

Further, in the electron transit layer 31, there is a region whichstarts from the interface between the buffer layer 20 and the electrontransit layer 31, and extends into the electron transit layer 31. Thethickness of this region is approximately 200 nm. This region is dopedwith a Fe concentration of approximately 2×10¹⁹ cm⁻³. Further, there isanother region 31 c in the electron transit layer 31. The region 31 calso starts from the interface between the buffer layer 20 and theelectron transit layer 31, and extends in the electron transit layer 31.The region 31 c is doped with a Fe concentration of approximately 1×10¹⁹cm⁻³ and is doped with a Si concentration of 2×10¹⁷ cm⁻³ or less. Thethickness of the region 31 c is in a range from 30 nm to 50 nm.

As a method of forming the regions doped with Fe and Si, there is amethod in which the supply amount of the source gas for Fe is increasedand SiH₄ for doping with Si is simultaneously supplied at each of thetimings when the regions are formed with Fe and Si doping. By doingthis, the semiconductor apparatus in this embodiment may bemanufactured. All the steps in this embodiment other than the above arethe same as those in the third embodiment.

Fifth Embodiment

Next, a fifth embodiment is described. In the semiconductor apparatusaccording to this embodiment, as illustrated in FIG. 16, doping not onlywith Fe but also with C is performed in the fifth embodiment.

To reduce remaining carriers, it is preferable to dope with Fe at higherconcentration. However, due to a difference in atomic radius, if Fe isinjected too much, the crystal property may be degraded. To overcome theproblem, in this embodiment, not only Fe but also C, which may functionsimilar to Fe, is injected. Specifically, Fe, which is injected into GaNcrystal, is substituted for Ga.

However, the covalent radius of Ga is 122 pm (1.22 {acute over (Å)}) andthe covalent radius of Fe is 132 pm (1.32 {acute over (Å)}). Therefore,the difference in the covalent radius between Ga and Fe is 10 pm. On theother hand, C, which is injected into GaN crystal, is substituted for N.The covalent radius of N is 72 pm (0.72 {acute over (Å)}) and thecovalent radius of C is 73 pm (0.73 {acute over (Å)}). Therefore, thedifference in the covalent radius between N and C is 1 pm.

Therefore, the difference in covalent radius between C and N is smallerthan that between Fe and Ga by one order of magnitude. Accordingly, whenan impurity element is doped into GaN, it is preferable to dope with Crather than with Fe to prevent the degradation of the crystal property.Therefore, in this embodiment, by doping with both Fe and C at the sametime, the dopant amount of Fe may be reduced and the degradation of thecrystal property may be prevented.

In this embodiment, the nucleation layer 11 is formed of AlN that isdoped with Fe at concentration of approximately 5×10¹⁷ cm⁻³. The firstbuffer layer 21 is formed of Al_(0.8)Ga_(0.2)N with Fe dopedconcentration of approximately 1×10¹⁸ cm⁻³. In the first buffer layer21, in the vicinity of the interface between the nucleation layer 11 andthe first buffer layer 21, there is formed a region 21 d that is dopedwith a Fe concentration of 2.5×10¹⁸ cm⁻³ and is doped with a Cconcentration of 7.5×10¹⁸ cm⁻³. The thickness of the region 21 d is in arange from 30 nm to 50 nm.

Further, the second buffer layer 22 is formed of Al_(0.5)Ga_(0.5)N withFe doped concentration of approximately 5×10¹⁸ cm⁻³. In the secondbuffer layer 22, in the vicinity of the interface between the firstbuffer layer 21 and the second buffer layer 22, there is formed a region22 d doped with a Fe concentration of approximately 1.25×10¹⁹ cm⁻³ andis doped with a C concentration of 3.75×10¹⁹ cm⁻³. The thickness of theregion 22 d is in a range from 30 nm to 50 nm.

Further, the third buffer layer 23 is formed of Al_(0.2)Ga_(0.8)N withFe doped concentration of approximately 1×10¹⁹ cm⁻³. In the third bufferlayer 23, in the vicinity of the interface between the second bufferlayer 22 and the third buffer layer 23, there is formed a region 23 dthat is doped with a Fe concentration of 2.5×10¹⁹ cm⁻³ and is doped witha C concentration of 7.5×10¹⁹ cm⁻³. The thickness of the region 23 d isin a range from 30 nm to 50 nm.

Further, in the electron transit layer 31, there is a region whichstarts from the interface between the buffer layer 20 and the electrontransit layer 31, and extends in the electron transit layer 31. Thethickness of this region is approximately 200 nm. This region is dopedwith a Fe concentration of approximately 2×10¹⁹ cm⁻³.

Further, there is another region 31 d in the electron transit layer 31.The region 31 c also starts from the interface between the buffer layer20 and the electron transit layer 31, and extends in the electrontransit layer 31. The region 31 d is doped with a Fe concentration ofapproximately 5.0×10¹⁹ cm⁻³ and is doped with a C concentration of1.5×10²⁰ cm⁻³. The thickness of the region 31 d is in a range from 30 nmto 50 nm.

As a method of forming the regions doped with Fe and C, there is amethod in which the supply amount of the source gas for Fe is increasedand CBr₄ for doping C is simultaneously supplied at each of the timingswhen the regions are formed where Fe and C are dopants.

Further, as another method, the layers may be formed by preparingconditions where C may become more likely to be introduced into thelayers by changing the supply amounts of the group-III source gas andthe group-V source gas while the supply amount of the source gas for Feis increased. By doing this, the semiconductor apparatus in thisembodiment may be manufactured. All the steps in this embodiment otherthan the above are the same as those in the third embodiment.

Sixth Embodiment

Next, a sixth embodiment is described. In the semiconductor apparatus inthis embodiment, the compositions in the buffer layer are changed not ina step-wise manner but continuously. Specifically, a buffer layer 120 isformed on the nucleation layer 11 as illustrated in FIG. 17.

In the buffer layer 120, in the direction from the nucleation layer 11to the electron transit layer 31, the composition ratio of Al isgradually decreased but the Fe concentration is gradually increased.Namely, the buffer layer 120 has composition slopes in which thecomposition ratio of Al is gradually decreased as the layer is formedand the Fe concentration is increased as the composition ratio of Aldecreases.

A part (a) of FIG. 17 illustrates an example configuration of thesemiconductor apparatus according to this embodiment, and a part (B) ofFIG. 17 schematically illustrates the changes in the composition ratioof Al and the Fe concentration.

As described above, by forming the buffer layer 120 so as to have thecomposition slopes, it may become possible to prevent the generation ofcarriers more effectively. Here, the configurations other than the aboveare the same as those in the first embodiment.

Seventh Embodiment

Next, a seventh embodiment is described. In this embodiment, asemiconductor apparatus having a gate isolation film and a protectionfilm is provided. Specifically, as illustrated in FIG. 18, a gateisolation film 61 is formed on the electron supply layer 32 and justunder the gate electrode 41. Further, a protection film 62 is formed onthe regions where the electron supply layer 32 is exposed. The gateisolation film 61 is formed of aluminum oxide (Al₂O₃), and theprotection film 62 is formed of silicon nitride (SiN).

Eighth Embodiment

Next, an eighth embodiment is described. In this embodiment, asemiconductor device, a power-supply apparatus, and a high-frequencyamplifier are described (provided).

The semiconductor device in this embodiment herein refers to adiscretely-packaged semiconductor apparatus according to the firstthrough seventh embodiments. The discretely-packaged semiconductorapparatus according to the first through seventh embodiments isdescribed with reference to FIG. 19. FIG. 19 schematically illustratesthe inside of the discretely-packaged semiconductor apparatus. However,the arrangement of the electrodes differs from that illustrated in thefigure in the first through seventh embodiments.

First, an HEMT semiconductor chip 410 using GaN-based semiconductormaterial is formed by cutting the semiconductor apparatus, which ismanufactured according to the first through seventh embodiments, bydicing. Then the semiconductor chip 410 is fixed on a lead frame 420with a die bonding agent such as solder. Here, the semiconductor chip410 corresponds to the semiconductor apparatus in the first throughseventh embodiments.

Next, a gate electrode 411 is connected to a gate lead 421 with abonding wire 431, a source electrode 412 is connected to a source lead422 with a bonding wire 432, and a drain electrode 413 is connected to adrain lead 423 with a bonding wire 433. Here, the bonding wires 431,432, and 433 are formed of a metal material. Further, in thisembodiment, the gate electrode 411 refers to the gate electrode padwhich is connected to the gate electrode 41 in the semiconductorapparatus according to the first through seventh embodiments.

Similarly, the source electrode 412 refers to the source electrode padwhich is connected to the source electrode 42 in the semiconductorapparatus according to the first embodiment. The drain electrode 413refers to the drain electrode pad which is connected to the drainelectrode 43 in the semiconductor apparatus according to the firstthrough seventh embodiments.

Next, resin-sealing is performed with a molded resin 440 by the transfermolded method. By doing this, it becomes possible to manufacture thediscretely-packaged semiconductor device of the HEMT using GaN-basedsemiconductor material.

Next, a power-supply apparatus and a high-frequency amplifier accordingto this embodiment are described. The power-supply apparatus and thehigh-frequency amplifier according to this embodiment refer to thepower-supply apparatus and the high-frequency amplifier using thesemiconductor apparatus according to the first through seventhembodiments.

First, with reference to FIG. 20, the power-supply apparatus accordingto this embodiment is described. The power-supply apparatus 460 includesa high-voltage primary circuit 461, a low-voltage secondary circuit 462,and a transformer 463 which is disposed between the primary circuit 461and the secondary circuit 462.

The primary circuit 461 includes an alternating-current (AC) source 464,a so-called “bridge rectifier circuit” 465, a plurality of switchingdevices (four switching devices in the example of FIG. 20) 466, aswitching device 467 and the like.

The secondary circuit 462 includes a plurality of switching devices(three switching devices in the example of FIG. 20) 468 and the like. Inthe example of FIG. 10, the semiconductor apparatus according to thefirst embodiment is used as the switching devices 466 and 467. Here, itis desired that the switching devices 466 and 467 in the primary circuit461 be normally-off semiconductors. As the switching devices 468 used inthe secondary circuit 462, typical metal insulator semiconductor fieldeffect transistors (MISFET) formed of silicon are used.

Next, with reference to FIG. 21, the high-frequency amplifier accordingto this embodiment is described. The high-frequency amplifier 470according to this embodiment may be used as, for example, a high-poweramplifier for a base station of cellular phones. The high-frequencyamplifier 470 includes a digital predistortion circuit 471, a mixers472, a power amplifier 473, and a directional coupler 474.

The digital predistortion circuit 471 compensates for non-lineardistortion of the input signals. The mixer 472 mixes the input signals,whose non-linear distortion has been compensated for, with an AC signal.The power amplifier 473 amplifies the input signal that has been mixedwith the AC signal. In the example of FIG. 21, the power amplifier 473includes the semiconductor apparatus according to the first embodiment.The directional coupler 474 performs monitoring on the input signal andthe output signal and the like.

In the circuit of FIG. 11, by a switching operation, the output signalmay be mixed with the AC signal by the mixer 472 and transmitted to thedigital predistortion circuit 471.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the inventionand the concepts contributed by the inventor to furthering the art, andare to be construed as being without limitation to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a showing of superiority orinferiority of the invention. Although the embodiment of the presentinventions has been described in detail, it is to be understood thatvarious changes, substitutions, and alterations could be made heretowithout departing from the sprit and scope of the invention.

What is claimed is:
 1. A semiconductor apparatus comprising: asubstrate; a buffer layer formed on the substrate; a first semiconductorlayer formed on the buffer layer; a second semiconductor layer formed onthe first semiconductor layer; and a gate electrode, a source electrode,and a drain electrode formed on the second semiconductor layer, whereinthe buffer layer is formed of AlGaN and doped with Fe, wherein thebuffer layer includes a plurality of layers having different Alcomponent ratios from each other, and wherein, when a first layer of thelayers is closer to the substrate than a second layer of the layers andthe second layer is closer to the first semiconductor layer than thefirst layer, the Al component ratio of the first layer is greater thanthe Al component ratio of the second layer and the Fe concentration ofthe first layer is less than the Fe concentration of the second layer.2. A semiconductor apparatus comprising: a substrate; a buffer layerformed on the substrate; a first semiconductor layer formed on thebuffer layer; a second semiconductor layer formed on the firstsemiconductor layer; and a gate electrode, a source electrode, and adrain electrode formed on the second semiconductor layer, wherein thebuffer layer is formed of AlGaN and doped with at least one of Fe, Si,and C, wherein, in the first semiconductor layer, a region which isdisposed on the buffer layer side is doped with Fe, and wherein a Feconcentration in the region is higher than a concentration of the atleast one of Fe, Si, and C in the buffer layer.
 3. The semiconductorapparatus according to claim 2, wherein the buffer layer includes aplurality of layers having different Al component ratios from eachother, and wherein, when a first layer of the layers is closer to thesubstrate than a second layer of the layers and the second layer iscloser to the first semiconductor layer than the first layer, the Alcomponent ratio of the first layer is greater than the Al componentratio of the second layer and the Fe concentration of the first layer isless than the Fe concentration of the second layer.
 4. The semiconductorapparatus according to claim 1, wherein a number of the plurality oflayers is three or more.
 5. The semiconductor apparatus according toclaim 4, wherein the plurality of layers are arranged such that the Alcomponent ratio decreases and the Fe concentration increases in adirection from a substrate side to a first semiconductor layer side. 6.The semiconductor apparatus according to claim 1, wherein, in each ofthe layers of the buffer layer, the Fe concentration in a region near aninterface between the layers is higher than the Fe concentration in anyregion other than the region near the interface.
 7. The semiconductorapparatus according to claim 6, wherein when a given layer of the layersis adjacent to another layer of the layers and a first region of thegiven layer faces a second region of the other layer via an interface,the Fe concentration of one of the first and second regions belonging toone of the given layer and the other layer that has a narrower band gapis greater than the Fe concentration of any other region in the one ofthe given layer and the other layer.
 8. The semiconductor apparatusaccording to claim 7, wherein the one of the first and second regions isdoped with Si as well as Fe.
 9. The semiconductor apparatus according toclaim 7, wherein the one of the first and second regions is doped with Cas well as Fe.
 10. A semiconductor apparatus comprising: a substrate; abuffer layer formed on the substrate; a first semiconductor layer formedon the buffer layer; a second semiconductor layer formed on the firstsemiconductor layer; and a gate electrode, a source electrode, and adrain electrode formed on the second semiconductor layer, wherein thebuffer layer is formed of AIGaN and is doped with at least one of Fe,Si, and C, and wherein, in the buffer layer, an Al component ratiogradually decreases and a concentration of at least one of Fe, Si, and Cgradually increases in a direction from a substrate side to a firstsemiconductor layer side.
 11. The semiconductor apparatus according toclaim 1, wherein the Fe concentration in the buffer layer is in a rangefrom 5×10¹⁶ cm⁻³ to 2×10²⁰cm⁻³.
 12. The semiconductor apparatusaccording to claim 1, wherein the substrate is a silicon substrate. 13.The semiconductor apparatus according to claim 1, wherein in the bufferlayer, one of the layers closest to the substrate is a nucleation layerformed of AIN.
 14. The semiconductor apparatus according to claim 1,wherein the first semiconductor layer is formed of a material includingGaN.
 15. The semiconductor apparatus according to claim 1, wherein thesecond semiconductor layer is formed of a material including AlGaN. 16.The semiconductor apparatus according to claim 1, wherein the bufferlayer, the first semiconductor layer, and the second semiconductor layerare formed by MOVPE (Metal Organic Vapor Phase Epitaxy).
 17. Thesemiconductor apparatus according to claim 1, further comprising: anisolation film formed between the second semiconductor layer and thegate electrode.
 18. A power-supply apparatus comprising: thesemiconductor apparatus according to claim
 1. 19. An amplifiercomprising: the semiconductor apparatus according to claim 1.